These switches may, for example, be used as single-pole multi-throw photo-switches, single-pole single-throw photo-switches, photo-transistors, photo-mixers, phase shifters or samplers. In the case of use as a sampler, a microwave transmission line is interrupted by a switch that is controlled by a reference optical signal and that switches between the on and off states at very precise times, defined by a clock, in order to sample a microwave electrical signal injected onto the microwave transmission line.
The invention most particularly applies to the microwave field. It will be recalled that the frequency band of microwave signals is from about a few gigahertz (GHz) to a few hundred GHz. Many applications in the fields of telecommunications and radars use such microwave signals. To perform the aforementioned functions in the microwave field, the switch must have a rapid response time of about one picosecond and a maximum contrast between the open state and the closed state.
It has recently been demonstrated that the performance of single-pole signal-throw microwave photo-switches may be increased (noise factor in particular decreased) by decreasing the dimensions of the line interruptions to dimensions of the order of magnitude of a micron, which makes the delivery of a sufficient amount of optical energy to the active zone of the switch more difficult. However, if the amount of light energy reaching the active zone of the switch is insufficient, the latter will have unacceptable ON/OFF switching times, and a low contrast between the 2 states of the switch. It is therefore sought to achieve maximum optical coupling between the optical control beam and the switch, i.e. to deliver the highest possible proportion of the emitted optical beam to the active zone.
Switching devices thus conventionally comprise optical components allowing the optical control beam to be conveyed to the active zone of the switch. These conveying means are interposed, on the path of the optical beam, between the optical source (a laser in general, for example a laser diode) and the active zone. These conveying means conventionally comprise a single-mode optical fibre allowing the optical control beam to be guided, and a convergent optical lens intended to focus the optical beam on the active zone. The optical components allow the optical control power to be optimized by focusing a maximum of light onto the active zone. For an optical beam of about one micron in size, and for an active zone of the same size, it is necessary to position the spot, formed by the optical beam on the optical component, with respect to the centre of the active zone of the switch with a precision of 0.1 microns.
However, it is not possible to adhesively bond these optical components to the switch. Specifically, such a contact induces a modification of the electromagnetic field leading to a modification of the transfer function of the microwave line of the switch. It is therefore necessary to preserve a sufficient distance, corresponding to the focal length of the lens, between the lens and the active zone, this making it more difficult to position the conveying means with respect to the active zone.
It is known to place a lensed optical fibre (optical fibre equipped with a convergent lens securely fastened to one end of the optical fibre) in a trench of V-shaped profile produced in a silicon substrate, facing an optoelectronic component comprising the microwave transmission line, which is also fastened to the substrate. The properties of the V (dimensions, inclination) are very precise since they are defined by the crystal properties of the silicon. This precision allows the precision in the desired relative position between the end of the lensed optical fibre and the active zone, in the plane of the active zone, to be obtained. The optimal position of the end of the optical fibre, along an axis perpendicular to the plane of the active zone, is obtained by measuring a maximum transmission coefficient of a signal conveyed by the microwave line through the switching device. The optical fibre is then definitively fastened to the carrier in the optimal position.
Another solution consists in adding a tool allowing the relative position of the optical fibre and the active zone to be adjusted, the optimal position being obtained by measuring the transmission coefficient on the microwave line through the switch, the tool allowing the optical fibre to be held for a sufficient time in the optimal position to allow the optical fibre to be fastened in this position with respect to the active zone.
However, control of the optimal position via the transmission coefficient is dependent on the frequency of the signal transmitted over the microwave transmission line. This measurement requires a microwave testbed equipped with a microwave-signal generator and a microwave-signal receiver to be provided.
Moreover, current optical devices deliver a focused beam of size larger than the active zone of the device in the plane of the active zone. The diameter of the beam is generally of the order of 2 microns in the active zone. The transmission maximum does not allow a sufficient precision to be obtained in relative position either in the plane of the active zone, or in the direction perpendicular to this plane.
Moreover, assuming that the relative position of the optical elements (for example an optical fibre equipped with a lens or lensed optical fibre) with respect to the active zone is unknown before adjustment, no indicator other than the maximum transmitted signal can be used to make the alignment between these two elements rapidly converge. There is then a risk of optical components, the lensed optical fibre for example, being damaged by contact with the device.